The conventional padlock comprises a body in which a lock is disposed such that a lock core of the lock can be turned by a key. The lock core is fastened with a retaining member rotatable in conjunction with the lock core. As the lock is positioned in a locking state, the retaining member is caused to rotate to position itself in a retaining position corresponding to the locking state, thereby causing the end of the arm of the shackle to be so retained that the end of the shackle arm can not be pulled out forcibly from the inside of the body of the padlock. In other words, the shackle arm of the conventional padlock is arrested by means of the rotatable retaining member. As a result, the conventional padlock can be easily tampered with by hitting violently the padlock body so as to cause the retaining member to turn involuntarily to release the shackle arm. In addition, the body of the conventional padlock must be hollow so that there is room for the retaining member to turn. Such hollow body of the conventional padlock is vulnerable to sabotage or burglary by means of a saw.